"The transition to that launched the evolution of animals from marks one of the most pivotal, and poorly understood, events in . Advances in and , and particularly the study of s, are yielding new insights into the biology of the unicellular progenitors of animals. and gene families critical for animal development (including s and s) evolved in protozoa before the origin of animals. Innovations in and expansions of certain gene families may have allowed the integration of cell behavior during the earliest experiments with multicellularity. The protozoan perspective on animal origins promises to provide a valuable window into the distant past and into the cellular bases of animal development."